1. Field of the Invention
The present invention generally relates to magnetic recording media and magnetic storage apparatuses, and more particularly to a magnetic recording medium and a magnetic storage apparatus which are suited for high-density recording.
2. Description of the Related Art
The recording density of longitudinal magnetic recording media, such as magnetic disks, has been increased considerably, due to the reduction of medium noise and the development of magnetoresistive and high-sensitivity spin-valve heads. A typical magnetic recording medium is comprised of a substrate, an underlayer, a magnetic layer, and a protection layer which are successively stacked in this order. The underlayer is made of Cr or a Cr-based alloy, and the magnetic layer is made of a Co-based alloy.
Various methods have been proposed to reduce the medium noise. For example, Okamoto et al., xe2x80x9cRigid Disk Medium For 5 Gbit/in2 Recordingxe2x80x9d, AB-3, Intermag ""96 Digest proposes decreasing the grain size and size distribution of the magnetic layer by reducing the magnetic layer thickness by the proper use of an underlayer made of CrMo, and a U.S. Pat. No. 5,693,426 proposes the use of an underlayer made of NiAl. Further, Hosoe et al., xe2x80x9cExperimental Study of Thermal Decay in High-Density Magnetic Recording Mediaxe2x80x9d, IEEE Trans. Magn. Vol. 33, 1528 (1997), for example, proposes the use of an underlayer made of CrTiB. The underlayers described above also promote c-axis orientation of the magnetic layer in a plane which increases the remanence magnetization and the thermal stability of written bits. In addition, proposals have been made to reduce the thickness of the magnetic layer, to increase the resolution or to decrease the width of transition between written bits. Furthermore, proposals have been made to decrease the exchange coupling between grains by promoting more Cr segregation in the magnetic layer which is made of the CoCr-based alloy.
However, as the grains of the magnetic layer become smaller and more magnetically isolated from each other, the written bits become unstable due to thermal activation and to demagnetizing fields which increase with linear density. Lu et al., xe2x80x9cThermal Instability at 10 Gbit/in2 Magnetic Recordingxe2x80x9d, IEEE Trans. Magn. Vol. 30, 4230 (1994) demonstrated, by micromagnetic simulation, that exchange-decoupled grains having a diameter of 10 nm and ratio KuV/kBT≈60 in 400 kfci di-bits are susceptible to significant thermal decay, where Ku denotes the magnetic anisotropy constant, V denotes the average magnetic grain volume, kB denotes the Boltzmann constant, and T denotes the temperature. The ratio KuV/kBT is also referred to as a thermal stability factor.
It has been reported in Abarra et al., xe2x80x9cThermal Stability of Narrow Track Bits in a 5 Gbit/in2 Mediumxe2x80x9d, IEEE Trans. Magn. Vol. 33, 2995 (1997) that the presence of intergranular exchange interaction stabilizes written bits, by MFM studies of annealed 200 kfci bits on a 5 Gbit/in2 CoCrPtTa/CrMo medium. However, more grain decoupling is essential for recording densities of 20 Gbit/in2 or greater.
The obvious solution has been to increase the magnetic anisotropy of the magnetic layer. But unfortunately, the increased magnetic anisotropy places a great demand on the head write field which degrades the xe2x80x9coverwritexe2x80x9d performance which is the ability to write over previously written data.
In addition, the coercivity of thermally unstable magnetic recording medium increases rapidly with decreasing switching time, as reported in He et al., xe2x80x9cHigh Speed Switching in Magnetic Recording Mediaxe2x80x9d, J. Magn. Magn. Mater. Vol. 155, 6 (1996), for magnetic tape media, and in J. H. Richter, xe2x80x9cDynamic Coervicity Effects in Thin Film Mediaxe2x80x9d, IEEE Trans. Magn. Vol. 34, 1540 (1997), for magnetic disk media. Consequently, the adverse effects are introduced in the data rate, that is, how fast data can be written on the magnetic layer and the amount of head field required to reverse the magnetic grains.
On the other hand, another proposed method of improving the thermal stability increases the orientation ratio of the magnetic layer, by appropriately texturing the substrate under the magnetic layer. For example, Akimoto et al., xe2x80x9cRelationship Between Magnetic Circumferential Orientation and Magnetic Thermal Stabilityxe2x80x9d, J. Magn. Magn. Mater. (1999), in press, report through micromagnetic simulation, that the effective ratio KuV/kBT is enhanced by a slight increase in the orientation ratio. This further results in a weaker time dependence for the coercivity which improves the overwrite performance of the magnetic recording medium, as reported in Abarra et al., xe2x80x9cThe Effect of Orientation Ratio on the Dynamic Coercivity of Media for  greater than 15 Gbit/in2 Recordingxe2x80x9d, EB-02, Intermag ""99, Korea.
Furthermore, keepered magnetic recording media have been proposed for thermal stability improvement. The keeper layer is made up of a magnetically soft layer parallel to the magnetic layer. This soft layer can be disposed above or below the magnetic layer. Oftentimes, a Cr isolation layer is interposed between the soft layer and the magnetic layer. The soft layer reduces the demagnetizing fields in written bits on the magnetic layer. However, coupling the magnetic layer to a continuously-exchanged coupled soft layer defeats the purpose of decoupling the grains of the magnetic layer. As a result, the medium noise increases.
Various methods have been proposed to improve the thermal stability and to reduce the medium noise. However, there was a problem in that the proposed methods do not provide a considerable improvement of the thermal stability of written bits, thereby making it difficult to greatly reduce the medium noise. In addition, there was another problem in that some of the proposed methods introduce adverse effects on the performance of the magnetic recording medium due to the measures taken to reduce the medium noise.
More particularly, in order to obtain a thermally stable performance of the magnetic recording medium, it is conceivable to (i) increase the magnetic anisotropy constant Ku, (ii) decrease the temperature T or, (iii) increase the grain volume V of the magnetic layer. However, measure (i) increases the coercivity, thereby making it more difficult to write information on the magnetic layer. In addition, measure (ii) is impractical since in magnetic disk drives, for example, the operating temperature may become greater than 60xc2x0 C. Furthermore, measure (iii) increases the medium noise as described above. As an alternative for measure (iii), it is conceivable to increase the thickness of the magnetic layer, but this would lead to deterioration of the resolution.
Accordingly, it is a general object of the present invention to provide a novel and useful magnetic recording medium and magnetic storage apparatus, in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a magnetic recording medium and a magnetic storage apparatus, which can improve the thermal stability of written bits without increasing the medium noise, so as to enable a reliable high-density recording without introducing adverse effects on the performance of the magnetic recording medium, that is, unnecessarily increasing the magnetic anisotropy.
Another object of the present invention is to provide a magnetic recording medium comprising at least one exchange layer structure, and a magnetic layer formed on the exchange layer structure, where the exchange layer structure comprises a ferromagnetic layer, and a non-magnetic coupling layer provided on the ferromagnetic layer and under the magnetic layer, and the ferromagnetic layer and the magnetic layer have antiparallel magnetizations. According to the magnetic recording medium of the present invention, it is possible to provide a magnetic recording medium which can improve the thermal stability of written bits, so as to enable reliable high-density recording without degrading the overwrite performance.
Still another object of the present invention is to provide a magnetic storage apparatus comprising at least one magnetic recording medium including at least one exchange layer structure and a magnetic layer formed on said exchange layer structure, and at least one head recording information on and/or reproducing information from the recording medium, where the exchange layer structure comprises a ferromagnetic layer and a nonmagnetic coupling layer provided on the ferromagnetic layer and under the magnetic layer, and the ferromagnetic layer and the magnetic layer have antiparallel magnetizations. According to the magnetic storage apparatus of the present invention, it is possible to provide a magnetic storage apparatus which can improve the thermal stability of written bits, so as to enable a reliable high-density recording without introducing adverse effects on the performance of the magnetic recording medium.